Central neuronal mechanisms responsive to pain have been described at spinal and supraspinal levels. The nature of these responses, how they interact, and what pathways and structures are implicated, remain only partially understood. There is a particular lack of knowledge about the central effects of deep and visceral pain and its interaction with somatosensory functions. A study of pain mechanisms at spinal and supraspinal levels, using endogenous peptides like bradykinin as natural and adequate pain stimuli for viscera and muscle, may lead to an improved understanding of the neurophysiology of pain and may facilitate the study of analgesic actions. Combined with selective lesions and the application of new anatomical techniques, like retrograde axonal transport of horseradish peroxidase, the structural foundations of central pain mechanisms can be established.